The present invention relates to a seagoing barge having a physical configuration and operative means which permit the barge to be partially submerged into bottom-supported gravity contact with the sea floor. More specifically, it relates to a barge and platform construction which can be submerged in a stable vertical path onto the sea floor.
The term "barge" as employed herein means a vessel which is capable of providing buoyancy for surface-floating transit as a first operational mode. The term "platform" is intended to mean a structure which rests on the sea floor and has a portion thereof elevated above the water surface. The barge disclosed herein has a surface-floating transit mode and a bottom-resting fixed position mode of operation.
Barges and platforms of the type above-referred-to are used for offshore oil exploration and production drilling purposes. When the barge/platform is to be utilized in benign sea areas a wide variety of platform configurations and erection techniques can be employed since low wave heights and ice-free weather conditions prevail. The barge/platform of the present invention can be used in such benign sea areas but can also be used in harsh sea areas such as those encountered in the arctic and antarctic latitudes.
When a barge/platform is to be used in arctic conditions such as those encountered in the Beaufort Sea, the Chukchi Sea, or the Bering Sea, specialized barge/platform designs must be employed. In many parts of the arctic latitudes the seas are generally covered with ice from October through June. A landfast ice cover begins to form in early October and grows seaward reaching a maximum thickness of approximately seven feet by May. Break-up usually begins in early June and continues throughout the remainder of the summer. This landfast ice consists of two distinct zones. The first zone extends about 15 miles outwardly from the shore to a water depth of about 33 feet. This zone usually consists of smooth first-year ice with a maximum thickness of seven feet. The second zone covers a water depth range of 33 to about 66 feet and contains a number of first-year pressure ridges. On some occasions, the ice in the second zone contains multi-year ice floes that are more consolidated than the first-year ice. Beyond the second zone and out to a position just past the Continental Shelf, at water depths exceeding 200 feet, is the transition zone which contains large pressure ridges that move around in sporadic motion. Beyond this zone is the permanent polar ice pack which is composed primarily of multi-year ice. This description of ice conditions holds particularly for the south part of the Beaufort Sea between Harrison Bay and the Prudhoe Bay.
Another environmental condition encountered in the Beaufort Sea area is that the sea floor consists mainly of pleistocene clays and holocene silts which have low force-bearing properties. As a result of this soft sea floor, some oil production equipment such as the well cap valve and tube systems known as "Christmas trees" have been known to sink into and disappear in the silt unless adequately buoyed.
The combination of ice floes on the surface of the sea and soft, low force-bearing, sea floor soil conditions presents special problems for barge/platform design and operations. The first problem is that the ice floes impact any support member extending through the water surface and this, in turn tends to push the entire platform off of its drilling location. The force of the ice floe has been calculated to be sufficient to move a large multiton platform. Due to the soft sea floor soil conditions such platforms cannot be adequately secured to the sea floor by piles or other economically feasible means. One suggested offshore structure in U.S. Pat. No. 4,048,943 to Gerwick is to employ a floating caisson which is maintained in position by mooring lines. In this structure, a cone-shaped surface is utilized as an ice breaking feature. However, many ice floes are sufficiently compact so that the mooring lines will break or the anchors will fail to hold before the ice floe is broken up by the conical surface.
Another form of barge/platform is shown in U.S. Pat. No. 4,080,796 which includes a ring shaped mat which is lowered into contact with the sea floor by the downward movement of three support legs. This vessel construction has two problems with respect to vessel operation. The first is that the ring-shaped bottom surface of the mat has an insufficient area of contact with clay and silt which are present on extensive areas of the sea floor to enable the development of large frictional contact so that the vessel will not be pushed off of location by the ice floes. This problem is accentuated because there are three support columns for contact with the ice floe. Thus, it is necessary in the operation of this platform to bury the ring-shaped mat below the sea floor in order to resist the lateral ice forces which develop overturning moments. Another problem is that the drilling operations must occur down through one of the support legs so that the drill string is not exposed to ice floes which are present beneath the upper hull. This requires then the placement of the drilling derrick over one of the legs which then creates an overturning moment about the center of the platform.
Barge/platform designs which utilize multiple support columns all encounter the problem of presenting multiple surfaces which would be impacted by ice floes in arctic seas such as described above. These designs have limited utility in such harsh sea conditions. Representative disclosures are shown in U.S. Pat. Nos. 2,873,581 to Hazak; 2,895,301 to Casagrande et al.; 2,953,904 to Christenson; 3,872,679 to Fischer; and 3,001,370 to Templeton.
Another type of offshore structure is set forth in representative U.S. Pat. Nos. 3,831,385 to Hudson et al.; 3,952,527 to Vinieratas et al.; 4,037,424 to Anders and 4,314,776 to Palmer et al. which shows bottom resting platforms which penetrate the water surface. These structures are very large and heavy and either do not have the necessary buoyancy for towing or only tow very poorly. These are not then barges. Also, difficulties are encountered in placing these structures at the drilling site. The difficulties arise due to two separate problems. The first is that, even when such strucutres are provided with sufficient buoyancy to float, the large volume of the structures submersed in the water creates high towing resistance. A second problem is that when the structure is submerged into bottom-resting contact with the sea floor the vessel can tilt and rapidly submerge into a bottom-contacting mode in which a large portion of the footing structure will submerge in the clay and silt sea floor. This will necessitate additional tug work for moving the structure to achieve a vertical attitude.
The problem of platform submergence at a high contact angle with the sea floor can be seen in U.S. Pat. No. 4,222,682 where container 1 and platform 111 are submerged from floating positions on the surface to sea floor contacting positions. If the sea floor is rocky and uneven, severe damage to the structures can occur from the high angle of impact shown in FIGS. 3, 14, and 15 of this patent.
Some offshore structures, mainly known as jack-up platforms, are towed to a use site and the bottom footing flooded to send it to the sea floor, then the top deck is jacked up out of the wave action. U.S. Pat. Nos. 3,996,754 to Lowery and 4,265,568 to Herrmann et al. are representative of this type. In some units of this type large flotation shells have been provided as in U.S. Pat. No. 3,086,367 to Foster and 4,142,819 to Challine et al. Operating problems occur during use of these shells since these are often flexibly connected to the main structure of the units to provide for movement relative thereto, and wave action on such large shells causes the connecting gear to break and foul. The dynamic loading forces generated on these large offshore structures by wave action can exceed the design limits of the connecting gear.
Yet other offshore structures require extensive construction work in order to erect the same, which often necessitates the use of mobile crane barges such as shown in FIG. 8 of U.S. Pat. No. 3,927,535 to Giblon
The mobile barge/platform of the present invention overcomes the above-described problems and provides a structure for offshore oil field development which has unique operational properties.